Cooling apparatus with laminar flow for electron permeable windows



I Dec. 23. T969 (3 w, sw s 3,486,060-

COOLING APPARATUS WITH LAMINAR mow FOR ELECTRON PERMEABLE WINDOWS 2Sheet heet 1 Filed Sept 6 1967 INVENTOR GEORGE w. ON BY nited StatesPatent 3,486,060 COQLING APPARATUS WITH LAMINAR FLOW FOR ELECTRONPERMEABLE WINDOWS George W. Swanson, Magnolia, Mass., assignor to HighVoltage Engineering Corporation, Burlington, Mass, a

corporation of Massachusetts Filed Sept. 6, 1967, Ser. No. 665,752 Int.Cl. 1191i 33/ 08 U.S. (11. 31374 6 Claims ABSTRACT OF THE DISCLOSUREBackground of the invention Various types of apparatus for coolingelectron transmissive windows in electron accelerators have been builtand used in the past. One such device is shown in U.S. Patent to D. M.Robinson, No. 2,820,165 issued Ian. 14, 1958 wherein there is describeda single jet for forcing cooled helium against the electron transmissivewindow. A second system is shown in the U.S. Patent to D. D. Miller No.2,898,492 issued Aug. 4, 1959. This patent teaches that if an elongatedair discharge slot is positioned adjacent to the window and largequantities of low velocity air are forced through this slot across thewindow face substantial cooling of the window is realized. Both of thesetechniques have been used successfully in the past.

However, with higher power electron beam accelerators and large windowareas these systems have been found lacking. The present invention wasdevised to overcome all the difliculties encountered in the prior artsystems and provides suitable cooling for higher beam energytransmission and for larger beam window areas. Moreover since thepresent invention is incorporated in and forms an integral part of thewindow frame the positioning of external apparatus is avoided. Thisassures that each window is cooled in the same manner and to the samedegree as any other window. This feature also eliminates the necessityof repositioning or dismounting of the cooling apparatus thus aiding inthe ease of repair or replacement of defective windows.

Summary of the invention The present invention relates to a coolingapparatus designed for cooling large area, electron transmissivewindows. Broadly speaking, the present invention accomplishes thisresult by providing along one side of the window a multiplicity ofangled in-line orifices positioned to direct high velocity,non-turbulent air streams across the electron, transmissive window facefor cooling purposes.

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A modification of the present invention provides for inter-connectingthe discharge ends of the orifices with a slot for difiusing the ejectedair streams so that laminar flow is achieved across a greater portion ofthe window area than was realized by prior art apparatus.

The invention further provides for means for generating at the windowside opposite the orifices a low pressure area to assist in and assurethe positive removal of the gas after its passage over the window. Thisfeature also removes any deleterious gases, such as ozone that may becreated by the beam after it has passed into the air, and reduces theturbulence normally encountered in these systems thus allowing for theprocessing of the material passing under the beam.

Brief description of the drawings FIGURE 1 is a perspective view of anelectron beam accelerator utilizing the invention;

FIGURE 2 is a sectional view of the window portion of the accelerator ofFIGURE 1;

FIGURE 3 is a planar view of a window retaining frame showing theorifices of the invention; and

FIGURE 4 shows a modification of the invention; and

FIGURE 5 shows the details of the modification depicted in FIGURE 4.

Description of the preferred embodiments Referring now to the drawingsand more particularly to FIGURE 1, there is shown schematically anelectron accelerator such as is manufactured by High Voltage EngineeringCorporation. This accelerator is capable of producing a highly energeticand narrow beam of electrons.

A cathode 10 produces a cloud of electrons (not shown) which are formedinto a beam and directed down an acceleration tube 11 through a flaredvacuum chamber 12 to Window 13. If the electrons have a suflicientenergy imparted thereto by their passage down tube 11 they will passthrough the electron transmissive window 13 which is comprised of arelatively thin foil of suitable material such as aluminum or titanium.Because such windows have a finite thickness they absorb part of theenergy of the beam and become heated. Such heating can cause localizedmelting or erosion with subsequent rupture of the window and decay ofthe vacuum in chamber 12. Thus cooling to a temperature below themelting point of the window material is required.

The present invention accomplishes such cooling in a highly reliable andeffective manner by forcing a thin laminar flow of air into contact withand across the face of the window such that optimum cooling is realized.

The detailed structure of the present invention required to accomplishthis result is shown in FIGURES 2 and 3. Referring simultaneously toboth these figures it is seen that the window edges are containedbetween a flange 14, provided on the vacuum chamber 12, and a windowframe 15 bolted to flange 14 by a plurality of bolts 18. To assure agood vacuum seal between the window, the frame and the flange a softgold or indium wire seal 16 is provided between flange 14 and window 13.The window frame 15 is of a shape and size to securely fit across themouth of chamber 12 and is usually rectangular with a centered elongatedopening 22 as shown in FIGURE 3. Extending along one long side 19 offrame 15 there is provided a multiplicity of in-line uniformly spacedorifices 20 generally of cylindrical shape and angled to direct a flowof air across the window face when the window is belled into the chamber12 under the combined influence of the vacuum in chamber 12, and theexternal atmosphere.

A typical frame has an overall length of approximately 26 inches and awidth of about 5 inches. The opening 22 in this frame is about 20 incheslong with a one inch radius on each end. This window will accommodate aneighteen inch beam. For effective cooling of a window aflixed in thisframe, 38 substantially evenly spaced orifices each of an inch indiameter are required.

Because the window 13 is depressed towards the cathode under theinfluence of the external atmosphere it is necessary, in order to assurea smooth laminar flow of gas across the window face, that these orificeseach be angled upwards with respect to the top surface 24 of the flame.Thus the entire line of orifices looks towards the concave window 13 ata very slight angle. It was empirically found that any angle between 25and 30 would provide a smooth laminar flow across the window face.

For ease of manufacture and attachment to the input plenum 25, blindpassageways 23, parallel to surface 24, connect orifices 20 to theplenum 25.

An air supply pipe 26 is connected between plenum 25 and an air supplysource such as a compressor (not shown). The compressor used may be forexample a Spencer turbo compressor manufactured by Spencer TurbineCompany. The only requirement for the compressor being that it becapable of effecting a laminar, low quantity, high velocity flow ofcooling gas across the window 13. When cooling gas is passed from thesource through the pipe 26 into plenum 25 an increase in the pressureoccurs in the plenum due to the constrictions caused by orifices 20.Thus when the cooling gas is forced into plenum 25 under the influenceof the compressor a resultant increase in pressure occurs as the gaspasses through orifices 20 to create a high velocity, low quantitycooling gas stream across the window face.

When the described window was exposed to a 3 ma./ in. 500 kv. beam, itwas found that optimum cooling occurred when 50 cubic feet per min. waspassed over the window with an orifice exit velocity of approximately3000 feet/min. Windows cooled with the described flow have lasted inexcess of 10,000 hours and have yet to exhibit any indications ofpotential failure. Based on these tests the reasonable life of windowscooled using the described quantities, velocities, etc. should be inexcess of 30,000 hours. Additional tests have shown that for larger beamtransmissive windows that the number of orifices, and that the quantityof the cooling gas must be proportionally increased. Thus, for example,a window capable of accommodating a 48 inch beam would require 114orifices, and

a quantity of 150 cubic feet per min. This increase in gas from eachorifice spreads out after it leaves the orifice such that the entireactive surface of the window, i.e. that potiron of the window throughwhich the beam passes, is covered by a thin layer of flowing gas.Because of the angle of and spacing between the orifices the flowing gasmolds itself to the window surface and transports away the heatgenerated in the window by the beam passing therethrough.

:Removal of the spent gas was found to be enhanced by contouring theinner face 30 of the opposite side 27 of the frame by beveling it suchthat the gas flowing over the window is not impeded when it reaches theexhaust side 27 of the window frame. The removal of the heated gas isfurther aided by providing positive exhaust means such as a manifold 28and an exhaust fan (not shown) along side 27. The positive exhaustaction of this manifold is increased by forming its remote edge 29 sothat it parallels the contour of inner face 30. The exhaust fan (notshown) which is coupled to the manifold 28 creates a subatmosphericpressure in the manifold which positively draws the spent gas and anydeleterious gases such as ozone that may be created by actionof the beaminto the manifold and away from the window face. This also permits thedisposal of such gases at a remote safe l0cation.

When processing bulky objects, such as panels, turbulence in the airstream is of little concern. However, when processing powders, liquids,thin films or the like it becomes vital that turbulence, which coulddisturb the material being processed, be eliminated. The describedmanifold with its contoured remote edge and positive exhaust eliminatesand prevents any such turbulence from occurring.

FIGURES 4 and 5 show different views of a modified embodiment of theinvention. This modification comprises the addition of a slot couplingthe orifices at their discharge end. Tests indicate that this slotimproves the cooling effect of the gas being forced over the windowface.

FIGURE 4 depicts a window frame 15A substantially the same as frame 15of FIGURE 3. This frame 15A also has 38 angled orifices 20A along itsinput side 19A and a window opening 22A. Again each orifice is of aninch in diameter and at an angle of between 25 and 30 with respect tothe top surface 24a. Each orifice 20A also intercepts a parallelpassageway 23a. The improvement is however found in the & inch wide slot35 which couples each orifice 20a to the adjacent orifice. This slotpermits a more uniform flow of gas across the window without asignificant loss of velocity.

It should be understood that slight variations in quantity of gas usedand in the exit velocity of the gas at the orifice mouth, can be madewithout going beyond the scope of the invention. Tests have indicatedthat velocities between 2800 ft./min. and 3200 feet are suitable andquantities between 40 c.f.m. and 60 c.f.m. can be used.

Having now described the present invention and various modifications andembodiments thereof, and since further embodiments may now becomeapparent to those skilled in the art it is desired that the inventiondescribed herein be limited only by the appended claims.

I claim:

1. In a high voltage electron accelerator apparatus comprising a cathodefor forming an electron beam, an accelerator for accelerating said beamthrough said apparatus, an electron transmissive exit window sealingsaid apparatus from the atmosphere and a frame for holding said windowon said apparatus, the improvement comprising means for effecting alaminar, low quantity, high velocity flow of cooling gas across saildwindow, said means comprising a plurality of in-line orifices in saidframe, a plenum coupled to said orifices and a compressor air supplypipe coupled to said plenum to supply a laminar, low quantity, highvelocity flow of cooling gas across said window to cool said windowbelow its melting temperature.

2. The apparatus of claim 1 wherein the exit portions of said orificesare at an angle with respect to the direction of said beam and saidwindow to direct said cooling gas against said window.

3. The apparatus of claim 2 wherein each of said orifices are spacedwith respect to the next adjacent orifice so that the gas flow emittingtherefrom blankets the electron beam transmissive portion of said windowwith a quantity of high velocity larninarly flowing cooling gassuflicient to maintain said window at a temperature below its meltingtemperature when said electron beam is passing through said window.

4. The apparatus of claim 1 wherein said means further comprises anexhaust manifold affixed to said frame to 5 6 extract the cooling gasafter it has flowed across said win- References Cited UNITED STATESPATENTS 5. The apparatus of claim 1 wherein the inner face of the sideof said frame parallel to said orifices is conto red 16 1 Mees 313 20 toprevent any impedance in said flow of gas across said 5 3375387 23g;tggz g a1 352 window.

6. The apparatus of claim 1 wherein said means further JAMES W,LAWRENCE, P ima E i er comprises an exhaust manifold having its remoteedge,

closest to the window, bent to parallel the inner face of LA FRANCHIAssistant Bummer the side of said frame parallel to said orificesthereby 10 S. Cl. X.R.

aiding in the removal of gases flowing over said faces. 44

